This invention relates to improvements in a beam processing system and a beam processing method for uniformly irradiating a beam of light, electrons, ions, or the like (particle beam) onto processing objects.
As a method of irradiating a beam of electrons, ions, or the like onto processing objects to thereby process them, there is known a method in which a plurality of processing objects are mounted on the same circumference of a rotary disk and, by rotating the rotary disk, a beam crosses the processing objects to scan them. In this method, the rotary disk is generally also reciprocated in its radial direction to thereby allow the beam to be irradiated over the entire surface of each processing object, which is called a mechanical scan. As a typical application example of such a mechanical scan, there is an ion implantation system for implanting ions into silicon wafers in the manufacturing process of semiconductor devices.
Referring to FIG. 1, a description will be given of particularly the rotary disk side in a mechanical scan type ion implantation system. A plurality of wafers (processing objects) 110 are mounted on the same circumference near the rim of a rotary disk 100. A scan direction by rotation of the rotary disk 100 and a scan direction by reciprocating movement (vertical direction in FIG. 1) of the rotary disk 100 are set perpendicular to each other. A beam 120 is fixedly irradiated at a specific position of a moving path of the wafers 110. By the combination of such two-direction scans, ion implantation is performed over the entire surface of each wafer 110. Normally, the rotational speed of the rotary disk 100 is sufficiently higher than the reciprocating speed of the rotary disk 100. Therefore, the scan by the rotation of the rotary disk 100 is called a high-speed scan, while the scan by the reciprocating movement of the rotary disk 100 is called a low-speed scan or a Y scan.
Since the rotary disk 100 is normally rotated at a constant speed (constant angular velocity), the scan speed of a high-speed scan increases in proportion to a radial distance R, where the beam hits on the rotary disk 100, as seen from the center of the rotary disk 100. Therefore, if a Y scan is simply performed with a uniform motion, the ion implantation density (concentration) becomes low at a portion where the scan speed of the high-speed scan is high, while the ion implantation density becomes high at a portion where the scan speed is low. For compensation thereof, the Y scan is slowed down at a portion where the high-speed scan becomes fast (i.e. the distance R is large), while, the Y scan is speeded up at a portion where the high-speed scan becomes slow (i.e. the distance R is small), thus achieving a uniform implantation amount (dose) by combining them. That is, the Y scan is performed by changing its speed so as to be inversely proportional to the radial distance R where the beam hits on the rotary disk 100.
The method of changing the speed of the Y scan in inverse proportion to the radial distance R of the rotary disk 100 as described above is called a (1/R) scan and employed in most batch-type ion implantation systems using a rotary disk [e.g. Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2006-60159].
The Y scan is repeated with a constant stroke. This stroke is defined between an outer overscan position and an inner overscan position. The outer overscan position is a position where beam irradiation is offset from the wafer 110 on the outer side of the rotary disk 100 due to movement of the rotary disk 100 downward in FIG. 1. The inner overscan position is a position where beam irradiation is offset from the wafer 110 on the inner side of the rotary disk 100 due to movement of the rotary disk 100 upward in FIG. 1. Referring to FIG. 1, assuming that the beam is initially located at the outer overscan position, the rotary disk 100, while being rotated, is driven upward until the beam reaches the inner overscan position. When the beam has reached the inner overscan position, the direction of the Y scan of the rotary disk 100 is reversed so that the rotary disk 100 is driven downward. When the beam has reached the outer overscan position, the direction of the Y scan of the rotary disk 100 is reversed so that the rotary disk 100 is driven upward. One reciprocating operation in which a beam starts from the outer overscan position and returns to the outer overscan position via the inner overscan position is given as one Y scan (one reciprocating scan).
On the other hand, for positioning control of the rotary disk 100, an initial position detection target portion 101 is provided at a predetermined position on the rotary disk 100 and a target portion sensor (not shown) for detecting the initial position detection target portion 101 is provided in the vicinity of the rim of the rotary disk 100. A detection signal from the target portion sensor is sent to a controller (not shown) having a function to control a motor which drives the rotary disk 100, and the controller uses this detection signal to implement positioning control of the rotary disk 100 [e.g. Patent Document 2: Japanese Patent (JP-B) No. 2909932].
The diameter of a wafer is mainly 200 mm or 300 mm. On the other hand, a beam normally has a circular cross section, but may have a flat cross section elongated in the horizontal direction. Hereinbelow, the diameter size in the case of the beam with the circular cross section and the vertical size in the case of the beam with the flat cross section will be collectively referred to as a “beam size”. In either event, since the beam size of the beam irradiated onto the wafers 110 is smaller than the diameter of each wafer, beam overlap irradiation like so-called overlap painting is carried out for achieving better uniformity of ion implantation. This is, in terms of one wafer, a method of causing regions of continuous twice beam irradiation on the wafer to partially overlap each other and is realized by performing a Y scan so that scan regions by a high-speed scan partially overlap each other. That is, if the beam is irradiated onto a partial region of the wafer at a certain rotation timing of the rotary disk 100, the beam is, at the next rotation of the rotary disk 100, irradiated onto the wafer so as to provide a region overlapping part of the above partial region on the wafer. Hereinafter, this overlap region will also be called a “beam overlap amount”.
The reason for employing such an overlap irradiation method is as follows. The (1/R) scan is ignored in the following explanation.
In the batch-type ion implantation system, when the rotary disk 100 is rotated at a low rotational speed reduced to half or less a normal high rotational speed, assuming that the Y-scan speed is equal to that at the time of the high-speed rotation, the distance of the Y-scan (scan pitch) moving during one rotation of the rotary disk 100 increases.
As the scan pitch during one rotation of the rotary disk 100 increases, the beam overlap amount in beam irradiation decreases. With respect to the beam size, as the beam size decreases, the beam overlap amount decreases. Then, if the scan pitch increases to be greater than a certain value or if the beam size decreases to be smaller than a certain value, the beam overlap amount decreases to be smaller than zero so that there is no overlap at all.
As shown in FIG. 1 as “High-Speed Rotation”, a scan pitch PH during one rotation of the rotary disk 100 is considerably small in the case of normal high-speed disk rotation (e.g. 800 to 1200 rpm) and, therefore, even if the beam size decreases, the beam overlap amount does not become zero unless the beam size becomes equal to or less than the scan pitch PH. Accordingly, even if the rotation start position of disk rotation is random every time a Y scan is started, no problem arises.
However, as shown in FIG. 1 as “Low-Speed Rotation”, in the case of low-speed disk rotation (e.g. 150 to 300 rpm), assuming that the Y-scan speed is equal to that in the case of “High-Speed Rotation”, a scan pitch PL during one rotation of the rotary disk 100 becomes large. In this case, under the conditions that the beam size is small and so on, possibility is expected that there is no beam overlap to cause occurrence of ion implantation unevenness or nonuniformity. Since the Y scan and the disk rotation are controlled independently of each other, it is considered that, in the case of the beam size that can achieve a certain beam overlap amount, if the Y scan is performed a plurality of times, ions are randomly implanted, so that the ion implantation unevenness finally disappears even in the case of the low-speed disk rotation.
However, as shown in FIG. 2, there occurs a case where the rotation start position of the rotary disk 100 at the start of a Y scan synchronizes (not “coincides”) or pseudo-synchronizes with the last rotation start position of the rotary disk 100. In this case, ion implantation is concentrated at certain portions of the wafers, so that there occurs a case where even if the Y scan is performed N times, the ion implantation uniformity is degraded to exceed 1%. This occurs particularly when a beam size Bs is not sufficiently large with respect to a scan pitch P (=Y-scan speed×disk rotation period), (P≧Bs), i.e. there is no beam overlap. FIG. 2 shows a transition of beam irradiation with respect to one wafer when a Y scan (reciprocating scan) is performed N times while the rotary disk 100 makes i rotations. If an irradiation state (state of the rotating wafer 110 observed at the same passing point) shown as “Final Implantation State” at the final stage in FIG. 2 is expressed in a plan view up to five Y-scan times, FIG. 6A is obtained, wherein regions with no beam overlap are formed. Naturally, FIG. 2 exaggeratingly shows the transition of beam irradiation for facilitating better understanding.
Of course, the foregoing problem is solved by reducing the Y-scan speed so as to produce beam overlap. However, in this case, there arises a new problem such as a problem of reduction in processing speed due to a reduction in Y-scan speed or a problem of rise in temperature of wafers due to prolongation of a beam irradiation time. Accordingly, there are also circumstances that cannot allow the Y-scan speed to be unlimitedly lowered.